The present invention relates to an underwater electrode assembly for contactless lithotripsy whereby particularly the tips of two electrodes face each other in close proximity to each other but with a clear and definite separation for establishing a spark gap.
At the present time it is a successful practice of long standing to comminute concrements in the body of human beings and in a manner which does not require invasive surgery and therefore does not require physical engagement with the concrement. Instead, acoustical shock waves are used which break up the concrement into sufficiently small pieces so that by natural discharge process these pieces (grit, dust) will be removed from the body. Shock waves for this purpose are produced by means of underwater spark discharges and the gap is arranged in the focal point of a water filled focusing chamber having a wall constructed to follow a rotational ellipsoid. As the spark jumps across the gap between the electrodes, a shock wave is produced. The energy is derived from the discharge of an electric capacitor.
Electrodes of the type that is suitable for such shock wave generation are known through German patent 26 35 635. Particularly FIG. 4 of that patent shows electrodes which are very satisfactory in practice. Generally speaking these electrodes include a tubular outer conductor which is continued in a yoke or cagelike configuration having an apex on which is mounted an electrode pointing inwardly into the cage. An inner conductor which traverses the first mentioned tubular outer conductor, but is electrically insulated therefrom, just ends in an electrode tip which faces the tip of the first mentioned electrode.
The generally coaxial configuration of the electrodes as described establishes a steady transition of the electrical field lines as they emerge from the inner conductor towards the outer conductor but also towards the cage and primarily to the tip of the first mentioned elecrode. The spark in the gap should, as much as possible, be produced directly along and in the electrode axis because that axis traverses precisely the focal point of the rotational ellipsoid. Initially this operation is quite certain. However, after so many uses the tips of the electrodes, no matter how refractory they are, will in fact begin to burn off which changes their contour in a basically unforseeable manner consequently the exact disposition of the arc whenever it occurs may randomly vary. This means that the arc may at times extend slightly off the focal points.
In addition certain misfirings will occur; initially only very rarely but as time progresses these misfirings may occur to an increasing extent which means that several plasma channels are established during a single discharge. The shock wave generation will become less and less concentrated and becomes somewhat fuzzy and only more or less in the vicinity of the focal point. In fact these misfirings can be acoustically ascertained; they are recognizable in contradistinction to the usual sharp sound as more muted beats. Needless to say that as soon as these various defects occur the electrodes have to be exchanged. An off-focus generation of the spark gap means that the refocusing of the shock waves as they have been reflected by the rotational ellipsoid, will not occur with a sufficiently high density and possible even off the second focus. That second focus is in the concrement to be comminuted. A slight off center ear may not be that dangerous if the concrement is of sufficient size but a reduction in shock wave density simply reduces the effectiveness of such shock wave. The breaking process is in fact impeded.